The present invention relates to a catalyst component for olefin polymerization, a catalyst and a process for producing polyolefin using said-catalyst. Particularly, the present invention provides an olefin polymerization catalyst having a high activity and having an ability of stably producing polyolefin without fouling on a polymerization reactor wall and the like by using an ion-exchange layered silicate having a specific structure. Further, the present invention provides a prepolymerization catalyst homogeneously prepolymerized, which has excellent powder properties and does not produce a residue of the catalyst leading to degradation of an outer appearance of a product.
A metallocene catalyst for olefin polymerization comprises a metallocene complex and a cocatalyst activating the metallocene complex. As the above cocatalyst, various compounds such as methyl aluminoxane, boron type compounds or the like have been proposed. In the production of polyolefin using a specific process, it is required to have a metallocene catalyst supported on a carrier in view of process properties and handling properties of polymer particles produced therefrom. Since a metallocene catalyst used by combining well known methyl aluminoxane and boron type compounds is often soluble in an organic solvent, it is used by being supported on an inorganic carrier such as silica or an organic carrier such as an organic polymer.
Also, a cocatalyst other than these cocatalysts has been reported. EP511665 discloses an example of producing an olefin polymer by using clay or clay mineral as an olefin polymerization catalyst component and combining it with a metallocene catalyst. In this catalyst system, the carrier is characterized by having a function of a cocatalyst activating the metallocene catalyst. Also, it is reported that an olefin polymerization activity is improved by using an ion-exchange layered compound treated by an acid, a salt or a combination with an acid and a salt as a catalyst component (EP683180).
On the other hand, it has been proposed to carry out prepolymerization for purposes of improving powder properties of a polymer obtained, preventing fouling of a polymerization reactor or preventing occlusion in a line of transporting a polymer after the polymerization reactor (JP-A-5-295022).
However, the above techniques must have been further improved, and it has been difficult to prevent production of fine polymer particles or agglomeration of polymer particles during polymerization. Particularly, in production of a low melting point polymer, these phenomena have been remarkably caused and there have been serious problems that an industrial scale production plant could not be operated continuously and stably for a long time. Further, according to the methods disclosed in these publications, a polymerization activity per solid catalyst component is not always satisfactory, and development of a catalyst satisfying both a polymerization activity and an operation stability has been demanded.
A first object of the present invention is to provide a catalyst system producing satisfactory polymer particle properties and particularly to provide a catalyst system satisfying both a high activity and satisfactory polymer particle properties. More particularly, in production of a low melting point polymer, it is demanded to satisfy the above requirements.
A second object of the present invention is to provide a catalyst system able to carry out a stable polymerization of a low melting point polymer. Generally, when polymerization is supported out by using the same catalyst under the same polymerization temperature conditions, powder properties are degraded as a melting point of a polymer produced becomes lower, and therefore there is a lower limit to a melting point of a polymer allowable to be industrially produced. The present invention is to provide a catalyst system capable of lowering this lower limit of a melting point of a polymer allowable to be produced.
A third object of the present invention is to provide a catalyst having a high upper limit to a polymerization temperature. When polymerization is carried out by using the same catalyst under such a polymerization conditions as to produce a polymer having the same melting point, particle properties of a polymer having a higher polymerization temperature are degraded, and therefore there is an upper limit to a polymerization temperature allowable to be industrially used. The present invention is to provide a catalyst system capable of improving this upper limit of a polymerization temperature industrially usable.
A fourth object of the present invention is to provide a prepolymerization catalyst homogeneously prepolymerized so as to reduce a residue of a catalyst leading to degradation of an outer appearance of a product.
The present inventors have variously studied, and have discovered that the above problems can be solved by using an inorganic silicate having a specific structure as a catalyst component for olefin polymerization. The present invention has been accomplished on the basis of this discovery.
That is, the present invention employs an ion-exchange layered silicate having the following properties as a carrier:
(a) having a specific pore size distribution; and
(b) having a carrier strength within a specific range.
Such a carrier as having these physical properties may be a material occurring in nature as far as having these properties (a naturally occurring material having these properties as it is has not been found by the present inventors up to now) or a material treated so specifically as to have these properties.
Examples of such treatments as to provide the aimed physical properties include:
(a) a chemical treatment illustrated below (particularly acid treatment) so as to be carried out under specific conditions to provide a specific pore size distribution;
(b) a granulation treatment carried out under specific conditions to provide a specific carrier strength; and
(c) a treatment with a specific organic aluminum compound. A more satisfactory effect can be expected by combining these treatments.
Another means for achieving the above-mentioned objects of the present invention is to use a prepolymerization catalyst having a specific structure for main polymerization. The specific structure means a structure wherein active precursor sites of a metallocene catalyst are uniformly dispersed within a particle of a prepolymerization catalyst. Since the metallocene catalyst has a high polymerization activity, it is important to efficiently remove reaction heat generated by polymerization. If the active precursor sites of the metallocene catalyst are not uniformly dispersed, heat is not satisfactorily removed at the part and a temperature is locally raised. As this result, there are caused problems that a produced polymer is dissolved in a solvent, that a produced polymer is melted or polymer particles are agglomerated to each other and that a produced polymer is adhered to a reactor wall.
In the present invention, it has been intensively studied as to how uniformly the active precursor sites should be dispersed in order to prevent the above-mentioned problems, and it has been discovered that it is possible to observe a dispersion state of active precursor sites of a metallocene catalyst in a particle by means of fluorescent analysis. According to the present invention, the above problems can be solved if an index showing a uniform dispersibility (prepolymerization homogenization index: H-value) is within a specific range.
Examples of means for making H-value within a specific range in the present invention include:
(a) to use an ion-exchange layered silicate having the above-mentioned specific structure; and
(b) to use specific prepolymerization conditions. A more satisfactory effect can be achieved by combining these operations.